ISSUE 4/2010

/// 04
Wang, Y.; Zhang, L.; Peng, L.; Ding, W.
Study of a permanent mold coating for magnesium alloy

A permanent mold coating for magnesium alloys has been investigated. The results show that the optimum composition of suspending agent (4 wt % sodium bentonite, 0.2 wt % carboxymethyl cellulose (CMC) and 0.3 wt % polyacrylamide) makes a suspending rate of 100 % by orthogonal experiment. Phosphate binder has better binding strength and antidesquamation property than silicasol binder. The fused magnesia powder with a compact microstructure has better binding strength and a lower requirement of phosphate binder than the light magnesia powder with a porous microstructure. The binding strength will be decreased obviously when inhibitor addition exceeds 6 wt %. Relationship between shearing stress and shearing rate can be described as: t1.093 = 14.1241.093 + 0.1861.093g1.093 and apparent viscosity vs. shearing rate, h0.17347 = (7.2342/g) 0.17347 + 0.000090.17347 using the Modified Casson model. Casson yield stress can be used to characterize the suspendability of the coating and for a suspending rate of 100 %, critical Casson yield stress is 14 Pa. Phosphate binder and inhibitor have no effect on the suspendability and Casson yield stress. Thermal conductivity vs. temperature shows linear relation and can be indicated as: l = -0.00497 + 7.91805 E-4 T (100~200 ℃), l = 0.15928 – 2.02116 E-4 T (200 to 450 ℃), l = 0.07475 + 1.79218 E-4 T (450 to 600 ℃).

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